IB Physics 12

Mr. Jean

December 5th, 2013

The plan:

• Video clip of the day

• Lenz’s Law Investigation

• Solenoids

• Important Definitions

• AC & DC engines

Faraday / Lenz’s Law:• Brightstorm: Faraday Lenz’s Law

– http://www.youtube.com/watch?v=qWu82nJS42I

Lenz’s Law Investigation

• Complete the Lenz’s Law investigation

• This mini-lab should only take about 30 minutes.

Lenz’s Law Mini-lab:

CHECK OUT THIS!

• This person has some excellent material with regards to E & M.

• http://www.youtube.com/user/lasseviren1/videos

Electro-magnetic Induction:

• Brightstorm: Electro Magnetic Induction: – http://www.youtube.com/watch?

v=BJcH7PELbqI

Lenz’s Law:

• http://www.launc.tased.edu.au/online/sciences/physics/Lenz%27s.html

• https://www.boundless.com/physics/induction-ac-circuits-and-electrical-technologies/magnetic-flux-induction-and-faraday-s-law/faraday-s-law-of-induction-and-lenz-law/

• http://hyperphysics.phy-astr.gsu.edu/hbase/electric/farlaw.html

Magnetic Field in a Solenoid:

• A long straight coil of wire can be used to generate a nearly uniform magnetic field similar to that of a bar magnet.

• Such coils, called solenoids, have an enormous number of practical applications.

• In the above expression for the magnetic field B, n is the number of turns per unit length, sometimes called the "turns density". The expression is an idealization to an infinite length solenoid, but provides a good approximation to the field of a long solenoid.

Solenoids:

• Brightstorm: Solenoids– http://www.youtube.com/watch?v

=r7EZb3MXJBM

DC Electric Motor

• Has a coil with an iron core ARMATURE⇒• It is surrounded by electromagnets

Problems with DC Power:

• 1. The magnetic forces are aligned directly opposite each other and will no longer experience a torque.– If you could change the direction of the

current, the coil would again experience a torque.

• 2. If the coil keeps turning, the leads will twist and eventually break.

AC Electric Motor

• Uses slip rings as commulator.

• Since the current is alternating, the motor will run smoothly only at the frequency of the sine wave.

• The magnetic field is sinusoidally varying, just as the current in the coil varies.

AC vs. DC Motors:

• Electric motors are mostly AC because our electric energy for industry and home is transmitted as AC.

• DC motor – starter motor on a car.

AC Electric Motor

• Uses slip rings as commulator.

• Since the current is alternating, the motor will run smoothly only at the frequency of the sine wave.

• The magnetic field is sinusoidally varying, just as the current in the coil varies.

AC vs. DC Motors:

• Electric motors are mostly AC because our electric energy for industry and home is transmitted as AC.

• DC motor – starter motor on a car.

AC Generators:

• Generators are essentially the same design as motors. – The mechanical energy input to a generator turns the

coil in the magnetic field. This produces an emf (voltage). A sinusoidal voltage output.

• The mechanical energy may come from:– i. Steam– ii. Wind– iii. Waterfall– iv. Electric motor

DC Generator: • The commutator must change the AC

flowing into its armature into DC.

• Commutators keep the current flowing in one direction instead of back and forth.

Power Production

• Generators were built by Tesla to generate electricity reliably and in large quantities.

• Most of today’s energy sold is in the form of AC because it can easily be transformed from one voltage to another.

High Voltage with Low Current:

• Power is transmitted at high voltages and low current without much energy loss (heating of wire) because it can be stepped down from the plant to many cities, to a city, to the household.

• Household typical outlet is 120 V AC.

Transformers: • Transformers are used to transfer energy

from one circuit to another by means of mutual inductance between two coils.

Types of Transformers:

• Transformers consist of a primary coil (input) and a secondary coil (output).

• Step-up Transformer – Secondary has more turns – greater electric potential (V) induced, caused

by the lowering for current (I)

Types of Transformers:

• Transformers consist of a primary coil (input) and a secondary coil (output).

• Step-down Transformer – primary has more turns– Thus less electric potential (V) induced

causing a greater current (I).

Transformers:

• Transferring energy from one coil to the other OR the rate of transferring energy is the power.

• The power used in the secondary is supplied by the primary.

• LAW OF CONSERVATION OF ENERGY